1. Field of the Invention
This invention generally relates to semiconductor integrated device design and fabrication and, more particularly, to the device isolation processes involving the local oxidation of silicon.
2. Description of the Related Art
The continuing trend of scaling down integrated circuits has shrunk the size of the devices on wafers to sub-half-micron dimensions and increased the circuit density to several million devices per chip. The manufacturability and reliability of these devices have recently received increasing attention. For a given chip size, an increase in the number of active circuit components requires that they be placed in close proximity to each other, thus forcing a corresponding reduction in the surface area of the circuit that can be occupied by electrical isolation structures. The trend for reducing the chip surface area consumed by electrical isolation structures, while maintaining the necessary electrical isolation of adjacent active components, has led to the development of several different isolation techniques.
The most common isolation fabrication technique is a process known as LOCOS (for LOCalized Oxidation of Silicon). In the LOCOS process, a silicon substrate is oxidized to form an isolation structure over the selected regions. These oxidized regions are known as field oxide regions and they are typically positioned so as to separate active areas of the semiconductor substrate where devices, such as transistors, will subsequently be formed.
In the conventional LOCOS technique, the process typically begins with the growth of a buffer layer, generally a thin pad oxide layer, over the substrate surface. The function of this layer is to prevent transition of stress between the silicon substrate and the subsequently deposited layers. Following this, one or more layers of masking material, typically including silicon nitride, are deposited on top of the pad oxide layer. Lithographic processes are used to define the nitride mask over active device regions of the substrate, while portions of the nitride layer are etched between the active device areas. Exposed regions of the substrate represent regions in which the field oxide (silicon dioxide) is to be thermally grown. Although the LOCOS process offers high reliability and proven high volume manufacturing compatibility, the effectiveness of this technique is limited by lateral encroachment during oxidation. The lateral oxide encroachment produces an effect commonly known as a bird's beak, the result of lateral diffusion of the oxidants under the nitride masking stack into the active device regions. The bird's beak manifests as a slowly tapering field oxide edge profile which penetrates into an adjacent usable active device area under the masking stack. As a result, the final width of the isolation structure is larger than the intended width and more of the active device regions are consumed.
These physical and electrical encroachments place severe restrictions on the use of LOCOS for ultra large scale integration (ULSI) applications. Specifically, as the device dimensions decrease to 0.5 .mu.m, the birds's beak encroachment on either side of the masking stack can penetrate under the masking stack and even meet, thereby eliminating the active area. This extended effect of bird's beak formation is known as isolation lifting, or bird's beak punchthrough, and it imposes important limitations on device packing density for sub-half-micron and ULSI applications.
Specifically, lateral diffusion of the oxidizing species under the masking stack in LOCOS applications becomes even more pronounced as the active area dimensions are decreased. As field oxidation proceeds in ULSI applications, the concentration of the oxidizing species quickly reaches supersaturation at the locations under the masking stack, thus causing the punchthrough effect. Furthermore, the narrower masking stack used in ULSI applications also contributes to this situation, since it is stiffer and will not deform against the edge lifting that accompanies the bird's beak penetration. Accordingly, the entire masking stack will lift during the field oxidation, thereby exacerbating the punchthrough effect.
Bird's beak encroachment is the most significant drawback to conventional LOCOS processes, preventing utilization of LOCOS for deep sub-micron applications and ULSI. This problem in standard LOCOS technology has motivated the development of many advanced variations of LOCOS isolation scheme for use in smaller device applications. These advanced processes, such as PELOX, NCL, RESSFOX, PAL, and RAL, focus attention on limiting the lateral diffusion of the oxidizing species under the nitride.
In general, these processes are known as spacer LOCOS processes, since they use spacer materials (poly, nitride or combined spacers) to passivate the edges of the active areas so as to suppress the lateral diffusion of the oxidizing species during the field oxide growth. Although the bird's beak problem is reduced by using spacers, the effectiveness of known spacer processes is limited, since adding spacers closes the Si region available for oxidation and can cause severe field oxide thinning effects. Also, the problem of consumption of spacers during the oxidation severely restricts the use of known spacer processes in fabricating very dense circuits.
Hence there is a need for a technique of forming isolation structures in semiconductor substrates wherein punchthrough of the isolation structures is minimized. To this end, there is a need for a modification of the standard LOCOS processing techniques and the spacer processing techniques that would prevent the punchthrough of isolation structures in sub-micron and ULSI applications.